I think it's a combination of a few factors.
First, there's a sort of ethical code among fansubbers that dictates that they fansub for "the love of anime" or somesuch, and that it's improprietous to profit off of fansubbing. I'm not sure how effective a reminder of this sort would be to people who don't subscribe to the same ethical code (i.e. the people at whom this message would be targeted), but there you have it.
Then, there's the belief that explicitly stating that you do not intend to profit off of your fansubs makes you less culpable for copyright infringement. I don't think this has been tested (since I don't know of any prosecutions of fansubbers), but I'm pretty sure it wouldn't hold up in court.
I suspect (but have no evidence) that this is a holdover from the pre-Internet days of fansub distribution when you actually needed to pass around video tapes to get your anime fix. I'd be interested to see what someone who participated in the pre-Internet days of fansubbing would have to say about this issue. [Update: C. Griffin's answer to this question contains an interesting perspective on the matter.]
Anyway, I haven't seen a disclaimer of this sort on any fansubs from at least the past five years or so - it's gone the way of the dinosaurs, presumably because everybody realized that it served no actual purpose.
Adam Davis also brought up in a comment another point I meant to address - fansubbers don't want other people profiting off of their work. So what's one way to do that? Slap a big "this is not for sale" warning on it, kind of like how a lot of free software comes with a notice saying something like "if you paid for this, you should ask for your money back because this is free software".
So here's the problem: basically all fansubbing these days is done with "softsubs" - that is, the subtitles are basically a text file that accompanies a video. It's dead easy to edit these things, so this is probably another reason that you don't see these disclaimers anymore: they're easy to remove. In the pre-softsub days (maybe pre-2005 or so? I don't have my chronology down for this very well), subtitling was done by re-encoding the video to have the subtitles baked in ("hardsubs"). This is much more difficult to alter, so disclaimers would have been more permanent (and hence, useful) back then.